Seattle
Genetics, Inc. (Nasdaq:SGEN) today will present interim phase 1
clinical data from SGN (News - Alert)-CD19A, an antibody-drug conjugate (ADC) in
development for the treatment of B-cell malignancies, at the American
Society of Clinical Oncology (ASCO) 50th Annual Meeting being
held May 30 to June 3, 2014 in Chicago, IL. SGN-CD19A is an ADC (News - Alert)
targeting CD19, a protein expressed on B-cell malignancies.

"We continue to make strong progress advancing our proprietary pipeline
programs, with SGN-CD19A representing one of five ADCs in clinical
development. SGN-CD19A targets a promising cancer marker expressed in a
variety of hematologic malignancies," said Jonathan Drachman, M.D.,
Chief Medical Officer and Executive Vice President, Research and
Development at Seattle Genetics. "These interim phase 1 data in
aggressive non-Hodgkin lymphomas presented at ASCO, along with previous
phase 1 data in acute lymphoblastic leukemia, demonstrate that SGN-CD19A
has encouraging antitumor activity with multiple complete remissions in
a patient population with significant unmet needs. The safety profile of
SGN-CD19A is generally manageable with a notable absence of significant
neuropathy or bone marrow suppression, which may enable future clinical
trials with novel combination regimens. We look forward to final results
from this ongoing phase 1 clinical trial to inform future SGN-CD19A
program development plans."

With over 16 years of experience and knowledge in ADC innovation,
Seattle Genetics is the leader in developing ADCs, a technology designed
to harness the targeting ability of antibodies to deliver cell-killing
agents directly to cancer cells. Of the more than 30 ADC candidates in
clinical development, greater than 20 utilize Seattle Genetics'
proprietary ADC technology. Seattle Genetics and its collaborators,
including Genentech, AbbVie and Progenics, have nine data presentations
at ASCO that highlight the widespread evaluation of its ADC technology
to potentially impact the way cancer is treated.

Data were reported from 37 patients with relapsed or refractory
aggressive B-cell non-Hodgkin lymphoma (NHL), including 32 patients with
diffuse large B-cell lymphoma (DLBCL), four patients with mantle cell
lymphoma (MCL (News - Alert)) and one patient with Grade 3 follicular lymphoma. The
median age of patients was 65 years and the mediannumber of prior
systemic therapies was two, with 10 patients (27 percent) having
received a prior autologous stem cell transplant. Among enrolled
patients, eight percent were primary refractory, 54 percent were
refractory to their last treatment and 38 percent had relapsed following
a response to their last treatment.

The primary endpoints of the ongoing clinical trial are to estimate the
maximum tolerated dose and to evaluate the safety of SGN-CD19A. In
addition, the trial is evaluating antitumor activity, pharmacokinetics,
progression-free survival and overall survival. In this dose-escalation
study, patients receive a single dose of SGN-CD19A on an every 3-week
basis. Key findings included:

No dose limiting toxicity was observed in the first cycle for any
patients. Adverse events were observed in the 6 milligrams per
kilogram (mg/kg) dosing regimen after the first cycle, therefore
enrollment was discontinued. The 3, 4 and 5 mg/kg cohorts are being
expanded, and the trial continues to enroll.

At the time of data analysis, of the 37 patients treated across all
dose levels, the objective response rate observed was 30 percent (11
patients). Six patients (16 percent) achieved a complete remission,
five (14 percent) achieved a partial remission, 13 (35 percent) had
stable disease and 13 (35 percent) had progressive disease as best
response. The clinical trial is ongoing with nine of the 37 patients
(24 percent) remaining on treatment and new patients continuing to be
enrolled.

More information about ongoing phase 1 SGN-CD19A clinical trials,
including enrolling centers, is available by visiting www.clinicaltrials.gov.

About SGN-CD19A

SGN-CD19A is an ADC comprised of an anti-CD19 monoclonal antibody linked
to a synthetic cytotoxic cell-killing agent, monomethyl auristatin F
(MMAF), using Seattle Genetics' industry-leading proprietary technology.
The ADC is designed to be stable in the bloodstream, and to release its
cytotoxic agent upon internalization into CD19-expressing tumor cells.
CD19 is expressed in B-cell ALL and NHL, including DLBCL. This approach
is intended to spare non-targeted cells and thus reduce many of the
toxic effects of traditional chemotherapy while enhancing the antitumor
activity.

Preclinical data presented at the 2011 American Association for Cancer
Research Annual Meeting demonstrated that SGN-CD19A effectively binds to
target cells, internalizes and induces potent cell-killing activity and
durable tumor regressions at low doses in multiple preclinical cancer
models. SGN-CD19A is being evaluated in two ongoing phase 1 clinical
trials for patients with B-cell ALL and aggressive NHL.

About Seattle Genetics

Seattle Genetics is a biotechnology company focused on the development
and commercialization of innovative antibody-based therapies for the
treatment of cancer. Seattle Genetics is leading the field in developing
antibody-drug conjugates (ADCs), a technology designed to harness the
targeting ability of antibodies to deliver cell-killing agents directly
to cancer cells. The company's lead product, ADCETRIS®
(brentuximab vedotin), is an ADC that, in collaboration with Takeda
Pharmaceutical Company Limited, is commercially available for two
indications in 40 countries, including the U.S., Canada, Japan and
members of the European Union. Additionally, ADCETRIS is being evaluated
broadly in more than 30 ongoing clinical trials. Seattle Genetics is
also advancing a robust pipeline of clinical-stage ADC programs,
including SGN-CD19A, SGN-CD33A, SGN-LIV1A, ASG-22ME and ASG-15ME.
Seattle Genetics has collaborations for its ADC technology with a number
of leading biotechnology and pharmaceutical companies, including AbbVie,
Agensys (an affiliate of Astellas), Bayer, Genentech, GlaxoSmithKline
and Pfizer. More information can be found at www.seattlegenetics.com.

Certain of the statements made in this press release are forward
looking, such as those, among others, relating to the therapeutic
potential of clinical product candidates. Actual results or developments
may differ materially from those projected or implied in these
forward-looking statements. Factors that may cause such a difference
include risk of adverse events as these ADCs advance in other clinical
trials. More information about the risks and uncertainties faced by
Seattle Genetics is contained in the company's 10-Q for the quarter
ended March 31, 2014 filed with the Securities and Exchange Commission.
Seattle Genetics disclaims any intention or obligation to update or
revise any forward-looking statements, whether as a result of new
information, future events or otherwise.